Solid oxide fuel cell

ABSTRACT

The invention relates to a solid oxide fuel cell having an inlet for fuel, an inlet for air, an outlet for product gas, an outlet for depleted air, a fuel flow path between the inlet for fuel and the outlet for product gas, an air flow path between the inlet for air and the outlet for air, and elements having an upstream part and a downstream part, wherein the upstream part has an anode layer, a cathode layer and an oxygen ion-conductive layer between the anode and the cathode layer, and wherein the downstream part has an oxygen ion- and electron-conductive layer. The invention further relates to a process for the generation of electricity and the production of carbon dioxide from a hydrocarbonaceous fuel using such a fuel cell.

The present invention relates to a solid oxide fuel cell comprising aplurality of elements each having an upstream part and a downstream partand to a process for the generation of electricity and the production ofcarbon dioxide using such a solid oxide fuel cell.

A solid oxide fuel cell is a fuel cell comprising a plurality of anodelayers and cathode layers separated from each other by means of a solidelectrolyte layer. The solid electrolyte is for example zirconia that isfully or partially stabilised with yttria. Charge transfer through thesolid electrolyte layer from the cathode to the anode is done by oxygenions.

The overall cathode reaction of a solid oxide fuel cell is:½O₂+2e ⁻→O₂ ⁻;and the overall anode reaction is:H₂+CO+O²⁻→H₂O+CO₂+2e ⁻.

The anode off-gas thus comprises carbon dioxide and water.

Typically in a tubular solid oxide fuel cell, off-gases, i.e. anodeoff-gas and oxygen-depleted air, are mixed and thus form a mixturecomprising a large amount of nitrogen and small amounts of carbondioxide, water and hydrogen. If however carbon dioxide could be obtainedin a highly concentrated form, preferably above 80 vol %, it can beefficiently liquefied and subsequently used in enhanced oil recovery orthe recovery of coal bed methane. Also for effective sequestration ofcarbon dioxide, a concentrated carbon dioxide stream is needed. Carbondioxide in lower concentration, e.g. 50 vol %, can usefully be appliedin the food and paper industry.

In WO 99/10945, a process for generating electricity using a tubularsolid oxide fuel cell in which process a stream rich in carbon dioxideis produced, is disclosed. In the process of WO 99/10945,oxygen-depleted air and anode off-gas are separately discharged from thesolid oxide fuel cell and the anode off-gas is oxidised in a ceramicafterburner to produce a stream mainly comprising carbon dioxide andwater. Water is then removed from this stream by condensation.

It has now been found that a ceramic afterburner can be integrated intoa solid oxide fuel cell. The plates or tubes of such an integrated fuelcell have an upstream part having a fuel cell function and a downstreampart having an afterburner function. An advantage of such an integratedsystem is that no means for separating the anode-off gas and theoxygen-depleted air are needed between the fuel cell part and theafterburner part of the system, since these gases are still separatedfrom each other when contacting the afterburner part of the system.

Accordingly, the present invention relates to a solid oxide fuel cellcomprising an inlet for fuel, an inlet for air, an outlet for productgas, an outlet for depleted air, a fuel flow path between the inlet forfuel and the outlet for product gas, an air flow path between the inletfor air and the outlet for air, and a plurality of elements each havingan upstream part and a downstream part, wherein the upstream partcomprises an anode layer, a cathode layer and an oxygen ion-conductivelayer between the anode and the cathode layer, and wherein thedownstream part comprises an oxygen ion- and electron-conductive layer.

The elements are either flat plates or tubes to form a planar or tubularsolid oxide fuel cell, respectively. Each element of the solid oxidefuel cell according to the invention has an upstream part having a fuelcell function and a downstream part having an afterburner function.Upstream and downstream is defined with respect to the flow of fuelduring normal operation, i.e. the upstream part is the part nearest tothe inlet for fuel and the downstream part is the part nearest to theoutlet for product gas.

The upstream part of each element has a composition, which is typicalfor a solid oxide fuel cell, i.e. an anode layer, a cathode layer and asolid electrolyte layer between the anode and the cathode layer. Anode,cathode and electrolyte layers for solid oxide fuel cells are know inthe art. The anode layer is a porous layer, usually composed of aceramic metal composite. A commonly used anode material comprises Ni andyttria-stabilised zirconia. The cathode layer is a porous layer of anelectron-conductive ceramic material, typically a mixed metal oxidehaving a perovskite structure. Lanthanum-strontium-manganese oxides area commonly used cathode material. The solid electrolyte layer of a solidoxide fuel cell is oxygen-ion conductive and has very limitedconductivity for electrons. This layer is dense and impermeable togases. Yttria-stabilised zirconia is commonly used.

In fuel cells, all elements, i.e. the tubes or plates, are electricallyconnected to each other. In the fuel cell according to the invention,all upstream parts of the elements, i.e. the parts having a fuel cellfunction, are electrically connected to each other by means known in theart.

The downstream part of each element comprises a layer having both oxygenion- and electron-conductivity, which layer is in the form of a denseceramic membrane and is impermeable to gases. This layer is furtherreferred to as mixed conductive layer. Such ceramic membranes are knownin the art. Examples are composites of metals and ceramic materials(cermets), bismuth oxides, and mixed oxides such as perovskites. Themixed conductive layer of the downstream part of each element may besupported on a porous ceramic layer.

Preferably, the fuel cell according to the invention is a tubular solidoxide fuel cell wherein each element is a tube. In the tubular fuel cellaccording to the invention, the upstream part of each tube has the anodelayer at its outside and the cathode layer at its inside. The tubespreferably have the shape of round cylinders, but oval tubes may beapplied. Instead of a plurality of tubes, the tubular fuel cell maycomprise one or more elongate monolithic structures having a pluralityof parallel, elongate channels. In the upstream part of the elongatemonolithic structure, the cathode layer is at the inside of each channeland the anode layer at the outside of the monolithic structure.

Preferably, each tube is closed at its upstream end. This means that theair to be fed to the cathode layer, i.e. to the inside of the tube orchannel, will be supplied via the downstream end of the tube or channel,counter-currently to the fuel flow.

In a first embodiment of the present invention, the first and the secondpart of each tube are distinct tubes that are connected to each other byhigh-temperature resistant gas-tight joints. The upstream anddownstream. part are connected in such a way that together they form atube. Since both parts of the tube are distinct, the composition andthickness of each part can be optimised for its function, i.e. fuel cellor afterburner function.

The joints may be any joints that can gas-tightly attach ceramic partsto each other under high-temperature conditions. Such joints are knownin the art and may for example comprise a ceramic O-ring combined with ametal flange.

In other embodiments of the invention, the cathode layer of the upstreampart and the porous support layer of the downstream part of each elementform a single plate or tube of the same ceramic material. The otherlayers, i.e. the electrolyte layer and the anode layer of the upstreampart and the mixed conductive layer of the downstream part are appliedto this porous single plate or tube by techniques known in the art, e.g.dipcoating, slib casting or plasma spraying. It is preferred to applyall these layers to the same side of the porous single plate or tube,since a continuous gas-impermeable layer can thus be formed by theelectrolyte and the mixed conductive layer in order to keep the gases inthe fuel flow path and the air flow path separated from each other.

During normal operation of the fuel cell according to the invention, ahydrocarbonaceous fuel is fed via the inlet for fuel to the anode sideof the elements. Air is fed via the inlet for air to the cathode side ofeach element. In the upstream part of the elements, the cathode andanode reactions take place resulting in the generation of electricityand the production of anode off-gas comprising hydrogen, carbon oxides,water and fuel at the anode side of the elements. Partially-depleted airis formed at the cathode side of the elements. Since the elements areimpermeable to gases, the anode off-gas flows to the downstream part ofthe element and will contact the mixed conductive layer at the surfacethat is facing the fuel flow path, i.e. the surface at the same side ofthe element as the anode layer. Oxygen, from the partially-depleted airformed at the cathode and/or from air directly supplied via the airinlet, will contact the mixed conductive layer at its opposite surface,i.e. the surface that is facing the air flow path which is the surfacethat is at the same side of the element as the cathode side.

The oxygen reacts with electrons to form oxygen-ions at the surface ofthe mixed conductive layer. The thus-formed oxygen-ions are transportedthrough the mixed conductive layer to the surface facing the fuel flowpath and react with the hydrogen, carbon monoxide and fuel in the anodeoff-gas to form water, carbon dioxide and electrons. The thus-formedelectrons are transported through the mixed conductive layer to thesurface facing the air flow path. Thus, product gas mainly comprisingcarbon dioxide and water is formed at the surface of the mixedconductive layer facing the fuel flow path and depleted air is formed atthe surface of the mixed conductive layer facing the air flow path.

Accordingly, the invention further relates to a process for thegeneration of electricity and the production of carbon dioxide from ahydrocarbonaceous fuel, wherein, in a solid oxide fuel cell ashereinabove defined:

-   a) air is contacted with the cathode layer and a hydrocarbonaceous    fuel or a partially reformed hydrocarbonaceous fuel is contacted    with the anode layer;-   b) by allowing the cathode and anode reactions to take place in the    upstream part of the elements, electricity is generated and an anode    off-gas comprising hydrogen, carbon oxides, water and fuel is formed    at the anode side, and partially-depleted air is formed at the    cathode side of the elements;-   c) the anode off-gas is reacted with oxygen ions at the surface of    the oxygen ion- and electron-conductive layer facing the fuel flow    path to form a product gas mainly comprising carbon dioxide and    water, and depleted air is formed at the surface of the oxygen ion-    and electron-conductive layer facing the air flow path.

The product gas and the depleted air are separately discharged from thefuel cell via the outlet for product gas and the outlet for air,respectively. In the tubular solid oxide fuel cell according to theinvention, the product gas and the depleted air may be kept separatedfrom each other by placing seals between the fuel and the air flow pathnear the tube outlets. Ceramic seals are examples of suitable seals. Itis advantageous to cool the gases before they are discharged from thefuel cell, since this makes sealing the fuel flow path from the air flowpath simpler.

Preferably, a gas stream rich in carbon dioxide is obtained by partiallycondensing the product gas and removing the condensed water from it. Thethus-obtained carbon dioxide rich gas stream may be used for enhancedoil recovery or recovery of coal bed methane.

The fuel may be any gaseous or vaporised hydrocarbonaceous fuel,preferably the fuel is a hydrocarbon stream that is gaseous at STPconditions (0° C. and 1 atm.) such as natural gas, methane, ethane orLPG, more preferably the fuel is natural gas.

The anode layer of a solid oxide fuel cell allows some internal steamreforming of hydrocarbons. Therefore, the hydrocarbonaceous fuel may bedirectly fed to the anode side of the fuel cell. It is, however,preferred that at least part of the fuel is pre-reformed to form mixturecomprising hydrogen and carbon monoxide prior to contacting it with theanode layer of the upstream part of the elements. Reference herein tofuel is to a hydrocarbonaceous fuel or to pre-reformed or partiallypre-reformed hydrocarbonaceous fuel.

The solid oxide fuel cell and the process according to the inventionwill be illustrated by means of FIGS. 1 to 3.

FIG. 1 schematically shows a longitudinal section of a tubular solidoxide fuel cell according to a first embodiment of the invention.

FIG. 2 shows a longitudinal section of a detail of a tube of the solidoxide fuel cell of FIG. 1.

FIG. 3 shows a longitudinal section of a detail of a tube of a solidoxide fuel cell according to a second embodiment of the invention.

In FIG. 1 is shown a solid oxide fuel cell 1 having an inlet for fuel 2,an inlet for air 3, an outlet for product gas 4, an outlet for depletedair 5 and a plurality of tubes 6. Only two tubes are shown. Each tube 6has an upstream part 7 and a downstream part 8 and is closed at theupstream end 9. The upstream 7 and the downstream part 8 of each tube 6are distinct tubes that are connected to each other by means of agas-tight joint 10.

During normal operation, air is supplied to the inside of each tube 6via the downstream end 11 of tube 6 by means of an air supply conduit 12having its outlet 13 in the upstream part 7 of the tube 6. In this way,the air is pre-heated before it contacts the cathode layer. Fuel is fedto fuel cell 1 via fuel inlet 2 and will react at the outside or anodeside of the upstream part 7 of tube 6 and the thus-formed anode off-gaswill flow to the outside of the downstream part 8 of tube 6. In thedownstream part of the tube, hydrogen, carbon monoxide and fuel in theanode off-gas will be oxidised to a product gas rich in carbon dioxideand steam. This product gas is discharged from fuel cell 1 via productoutlet 4. Depleted air is discharged via outlet 5. Seal 14 keeps theproduct gas and the depleted air separated from each other.

In FIG. 2 is shown part of tube 6 of the solid oxide fuel cell ofFIG. 1. Line L is the longitudinal axis of tube 6. The layered structureof the upstream part 7 of tube 6 and joint 10 connecting the upstream 7and the downstream part 8 of tube 6 to each other are shown in moredetail. The upstream part 7 of tube 6 has an anode layer 15 at theoutside, a cathode layer 16 at the inside and a solid electrolyte layer17 between the anode and the cathode layer. The downstream part 8 oftube 6 is distinct from the upstream part 7 and has a single layer 18 ofceramic material, which is a mixed conductive layer. Joint 10 providesfor a gas-tight connection between the upstream 7 and the downstreampart 8 of tube 6. Joint 10 is formed by the combination of ceramicO-ring 19 and metal flange 20.

FIG. 3 shows part of tube 6 of solid oxide fuel cell 1 of a secondembodiment of the invention. In this embodiment, the cathode layer 16and the porous support layer 21 of the downstream part 8 of tube 6 forma single tube and are composed of the same ceramic material. The mixedconductive layer 18 is located on the outside of the support layer 21,such that the electrolyte layer 17 and the mixed conductive layer 18form a continuous gas-impermeable layer to prevent gases to pass fromthe outside to the inside of the tube.

1. A solid oxide fuel cell comprising an inlet for fuel, an inlet forair, an outlet for product gas, an outlet for depleted air, a fuel flowpath between the inlet for fuel and the outlet for product gas, an airflow path between the inlet for air and the outlet for air, and aplurality of elements each having an upstream part and a downstreampart, wherein the upstream part comprises an anode layer, a cathodelayer and an oxygen ion-conductive layer between the anode and thecathode layer, and wherein the downstream part comprises an oxygen ion-and electron-conductive layer.
 2. The solid oxide fuel cell of claim 1,wherein the downstream part of the elements further comprises a porouslayer supporting the oxygen ion- and electron-conductive layer.
 3. Thesolid oxide fuel cell of claim 2, wherein the cathode layer of theupstream part of the element and the porous layer supporting the oxygenion- and electron-conductive layer of the downstream part of the elementare the same layer.
 4. The solid oxide fuel cell of claim 3, wherein theelements are flat plates.
 5. The solid oxide fuel cell of claim 1,wherein the elements are tubes and the anode layer is located at theoutside of the upstream part of the tube and the cathode layer at theinside of the upstream part of the tube.
 6. The solid oxide fuel cell ofclaim 5, wherein the upstream part and the downstream part of each tubeare distinct tubes that are connected to each other by means of ahigh-temperature resistant, gas-tight joint.
 7. The solid oxide fuelcell of claim 5, wherein each tube is closed at its upstream end.
 8. Aprocess for the generation of electricity and the production of carbondioxide from a hydrocarbonaceous fuel, wherein, in a solid oxide fuelcell comprising an inlet for fuel, an inlet for air, an outlet forproduct gas, an outlet for depleted air, a fuel flow path between theinlet for fuel and the outlet for product gas, an air flow path betweenthe inlet for air and the outlet for air, and a plurality of elementseach having an upstream part and a downstream part, wherein the upstreampart comprises an anode layer, a cathode layer and an oxygenion-conductive layer between the anode and the cathode layer, andwherein the downstream part comprises an oxygen ion- andelectron-conductive layer, the process comprises: a) contacting thecathode layer with air; and, contacting a hydrocarbonaceous fuel or apartially reformed hydrocarbonaceous fuel with the anode layer; b)allowing the cathode and anode reactions to take place in the upstreampart of the elements; generating electricity and an anode off-gascomprising hydrogen, carbon oxides, water and fuel; and, formingpartially-depleted air at the cathode side of the elements; c) reactingthe anode off-gas with oxygen ions at the surface of the oxygen ion- andelectron-conductive layer facing the fuel flow path to form a productgas mainly comprising carbon dioxide and water; and forming depleted airat the surface of the oxygen ion- and electron-conductive layer facingthe air flow path.
 9. The process of claim 8, further comprisingpartially condensing the product gas; and, removing water from it toproduce a stream rich in carbon dioxide.
 10. The process of claim 8,further comprising converting at least part of the hydrocarbonaceousfuel into a mixture comprising hydrogen and carbon monoxide prior tocontacting it with the anode surface of the upstream part of theelements.
 11. The solid oxide fuel cell of claim 2, wherein the elementsare tubes and the anode layer is located at the outside of the upstreampart of the tube and the cathode layer at the inside of the upstreampart of the tube.
 12. The solid oxide fuel cell of claim 3, wherein theelements are tubes and the anode layer is located at the outside of theupstream part of the tube and the cathode layer at the inside of theupstream part of the tube.
 13. The solid oxide fuel cell of claim 12,wherein the upstream part and the downstream part of each tube aredistinct tubes that are connected to each other by means of ahigh-temperature resistant, gas-tight joint.
 14. The solid oxide fuelcell of claim 6, wherein each tube is closed at its upstream end. 15.The process of claim 9, further comprising converting at least part ofthe hydrocarbonaceous fuel into a mixture comprising hydrogen and carbonmonoxide prior to contacting it with the anode surface of the upstreampart of the elements.
 16. The process of claim 9, wherein in the solidoxide fuel cell the downstream part of the elements further comprises aporous layer supporting the oxygen ion- and electron-conductive layer.17. The process of claim 16, wherein the cathode layer of the upstreampart of the element and the porous layer supporting the oxygen ion- andelectron-conductive layer of the downstream part of the element are thesame layer.
 18. The process of claim 17, wherein in the solid oxide fuelcell the elements are flat plates.
 19. The process of claim 9, whereinin the solid oxide fuel cell the elements are tubes and the anode layeris located at the outside of the upstream part of the tube and thecathode layer at the inside of the upstream part of the tube.
 20. Theprocess of claim 9, wherein in the solid oxide fuel cell, the upstreampart and the downstream part of each tube are distinct tubes that areconnected to each other by means of a high-temperature resistant,gas-tight joint.